In a magnetic resonance imaging system (abbreviated to “magnetic resonance system”), the body to be examined may be exposed to a relatively high main magnetic field (the so-called “B0 field”), for example, of 1.5, 3, 7 Tesla or more, with the aid of a main magnetic field system. Radiofrequency excitation signals (e.g., RF signals) are, via a radiofrequency transmission system, emitted by suitable antenna apparatuses with one or more transmission antenna elements. This is intended to lead to the nuclear spins of specific atoms or molecules, which are excited in a resonant fashion by this radiofrequency field, being tilted by a defined flip angle in relation to the magnetic field lines of the main magnetic field. The resulting flip angle distribution is also referred to as nuclear magnetization or abbreviated to “magnetization” in the following text. When the excited nuclear spins relax, radiofrequency signals, (so-called magnetic resonance signals), are emitted. The radiofrequency signals are received by suitable reception antennas and processed further. Timed to coincide with this, a gradient magnetic field is additionally superposed on the main magnetic field for selective excitation or for spatial encoding of the magnetic resonance signals with the aid of a gradient system. The desired image data may be reconstructed from the raw data acquired. In conventional systems, the emission of the radiofrequency signals (e.g., the so-called B1-field) for nuclear spin magnetization may be brought about by a so-called “whole body coil” (e.g., “body coil”) that is securely arranged around the measurement space (patient tunnel). The magnetic resonance signals may be received with the aid of so-called local coils that are positioned more closely to the body of the patient. In principle, magnetic resonance signals may also be received by the whole body coil and/or the RF signals may be transmitted by the local coils.
For a specific measurement, a magnetic resonance system actuation sequence (also abbreviated to “actuation sequence”) with a radiofrequency pulse train to be emitted (e.g., RF pulse train, in particular, a sequence of excitation signals in time) and a gradient pulse train may be switched in a manner coordinated therewith for controlling the gradient system (e.g., with appropriate gradient pulses in the slice selection direction, in the phase encoding direction and in the readout direction or frequency encoding direction, often in the z-direction, y-direction, and x-direction) and further control prescriptions is generated in advance. To this end, a multiplicity of parameters and control prescriptions for the actuation sequence are defined in a so-called measurement protocol or control protocol. By way of example, this measurement protocol may be recalled from a memory for a specific measurement and may be modified by the user in situ. During the measurement, the magnetic resonance system may be controlled fully automatically on the basis of this actuation sequence, where the control apparatus of the magnetic resonance system reads and works through commands from the measurement protocol.
In order to generate the actuation sequences, of an RF pulse train in particular, a target magnetization, e.g., a desired spatial flip angle distribution, may be prescribed (e.g., by the measurement protocol and/or by the user). An appropriate RF pulse train is calculated by a suitable RF pulse optimization program, which may operate using a numerical optimization method using a target function to be optimized or minimized such that this target magnetization is obtained.
To this end, current “field distribution maps,” (e.g., field distribution maps taking into account the current examination object and the current examination arrangement), may be required. These field distribution maps include, in particular, the “B1-maps,” which in each case may specify the spatial field distribution for a specific transmission antenna element or for a combination of transmission antenna elements. The field distribution may be specified in uT/V as a function of the location {right arrow over (r)} , e.g., the field strength per RF amplitude. In particular, the spatial sensitivity describes the spatial dependence of the flip angles generated by the transmission antenna element or the combination in the region of the measurement space (e.g., in particular over the so-called “field of view” of the magnetic resonance imaging system). Therefore, the B1-maps directly or indirectly describe the circular component of the B1-field (e.g., the so-called B1+-field) that rotates in the circumferential direction of the nuclear magnetization. Another type of field distribution maps is the so-called “B0-maps,” in which the spatial distribution of the main magnetic field (e.g., B0-field) is reproduced, in particular, also occurring inhomogeneities.
The field distribution maps are taken into account in the optimization method in order to find the ideal actuation sequence for the measurement to be carried out for the current examination object in the current examination surroundings.
The information for the B1-maps is often used in the target function to avoid inhomogeneities or geometric distortions of the B1-field or of the B1+-field, for example, due to radiofrequency shimming (e.g., B1+-shimming), in a spatially selective excitation by the transmission antenna elements. Falsifications, caused thereby, of the raw data for the magnetic resonance images may often be eliminated or at least strongly reduced. Knowledge of the spatial sensitivity of the affected transmission coils or transmission antenna elements with respect to the current examination object may be an important demand for being able to calculate ideal pTX-RF pulse sequences. This knowledge may be particularly useful in the case of so-called parallel transmission methods (e.g., pTX methods), in which radiofrequency pulses (which in combination form the radiofrequency pulse train) are emitted by a plurality of independently actuatable transmission channels or transmission antenna elements, and which then superpose in the measurement space in order to obtain an individually definable radiofrequency field.
Since the sensitivity of each transmission antenna element or transmission channel is required for each slice or for each image volume, the acquisition of the field distribution maps, in particular of the B1-maps that may be used for the aforementioned optimization method, is relatively time consuming and substantially increases the overall examination duration within clinical routine.